MySQL如果不存在就更新的三种方法（含不受主键限制）

 网络上找了下关于MySQL如果不存在就更新的语句，全部都是通过主键对一条记录做插入修改的。这里记录下不受主键限制的语法：

INSERT INTO wp_postmeta (post_id, meta_key) SELECT ?id, ‘page_title’  FROM DUAL WHERE NOT EXISTS (SELECT meta_id FROM wp_postmeta WHERE post_id = ?id AND meta_key = ‘page_title’);

UPDATE wp_postmeta SET meta_value = ?page_title WHERE post_id = ?id AND meta_key = ‘page_title’;

顺便也记录下主键限制的语法：

INSERT INTO users (user_id, name, email)

VALUES (1, 'John Doe', 'johndoe@example.com')

ON DUPLICATE KEY UPDATE email = 'johndoe@example.com';

还有一种方法是通过REPLACE

REPLACE INTO users (user_id, name, email)

VALUES (1, 'John Doe', 'johndoe@newdomain.com');